The present invention relates to a heterostructure bipolar transistor using a heterojunction as an emitter-base junction.
A conventional bipolar transistor has an npn or pnp structure wherein emitter, base, and collector layers are made of a common semiconductor material. In this case, emitter and collector junctions are each a homojunction.
Bipolar transistors using a heterojunction as the emitter junction and/or collector junction are receiving a great deal of attention and are being extensively studied these days. The heterojunction bipolar transistor has an advantage in that, when the emitter layer is made of a semiconductor material having a wider energy gap than that of the base layer, the emitter injection efficiency can be enhanced. When the emitter junction is forward biased, carriers can be easily injected from the emitter to the base while carrier injection from the base to the emitter is limited due to an energy gap difference between the emitter and base layers. Therefore, a current gain of the heterostructure bipolar transistor becomes higher than that of the conventional homostructure type.
The principle of the heterostructure bipolar transistors has been conventionally known. Recently, several examples have been published. FIG. 1 shows the basic structure of conventional heterostructure bipolar transistors wherein a heterojunction is used as the emitter junction. This transistor comprises a GaAs-GaAlAs structure. An n-type GaAs collector layer 2, a p-type GaAs base layer 3, and an n-type Ga.sub.1-x Al.sub.x As emitter layer 4 are sequentially deposited on an n.sup.+ -type GaAs substrate 1. Reference numeral 5 denotes a collector electrode; 6, a base electrode; and 7, an emitter electrode. The emitter layer 4 comprises an n.sup.- -type first emitter layer 4a which is in contact with the base layer 3, and an n.sup.+ -type second emitter layer 4b which is in contact with the emitter electrode 7. A common feature of conventional heterostructure bipolar transistors is in that the first emitter layer 4a is thick. The structure in which the emitter layer comprises a two-layer structure of high- and low-impurity concentration layers and the first emitter layer is thick aims at decreasing the emitter junction capacitance so that a switching speed is increased (e.g., H. Kroemer "Heterostrucure Bipolar Transistors and Integrated Circuits", Proc. IEEE, Vol. 70, No. 1, PP. 13-25, January 1982). In fact, when the low-impurity concentration layer has a sufficient thickness in a one-sided abrupt junction constituted by such high- and low-impurity concentration layers, it is known that a junction capacitance C.sub.JE can be expressed, in terms of an impurity concentration N.sub.E of the low-impurity concentration layer, as follows: EQU C.sub.JE "N.sub.E.sup.1/2
In order to clarify the subsequent discussion, the concept of the switching speed of a transistor will be clarified. In general, the switching operation of transistors includes turn-on and turn-off. A propagation delay time t.sub.pd, the average value of a turn-on time t.sub.on and a turn-off time t.sub.off, is taken as a reference of the switching speed. When an input signal of FIG. 2 is supplied to a transistor, the turn-on time t.sub.on is defined to be a time for an outut current to increase from 0% to 50%, and the turn-off time t.sub.off is defined to be a time for the output current to decrease from 100% to 50% (see FIG. 3).
The present inventors have made extensive studies of thicknesses of the respective layers and the relationship between the impurity concentration and the switching speed of the heterostructure bipolar transistor shown in FIG. 1, by means of a numerical analysis model (e.g., M. Kurata "Principles of Operation of Bipolar Transistors", 1980, Kindai Kagaku-sha, and M. Kurata, "Numerical Analysis for Semiconductor Devices", 1982, Lexington Books, D. C. Heath and Company). The present inventors have reached an opposite conclusion to the conventional theory. In particular, they found that the switching speed of the conventional transistor (to be referred to as type A hereafter) having the first emitter layer of a low-impurity concentration and a large thickness was greatly lower than that of a transistor (to be referred to as type B hereinafter) having only a single high-impurity concentration layer (omitting the first emitter layer). The results are shown in Table 1.
TABLE 1 ______________________________________ type A B ______________________________________ V.sub.on (V) 1.5 1.3 J.sub.E (A/cm.sup.2) 1.25 .times. 10.sup.4 1.01 .times. 10.sup.4 J.sub.C (A/cm.sup.2) 8.90 .times. 10.sup.3 9.59 .times. 10.sup.3 t.sub.on (psec) 13.0 3.8 t.sub.off (psec) 159 14.5 t.sub.pd (psec) 86 9.2 ______________________________________
In this numerical analysis, conditions are given in the circuit shown in FIG. 4 such that a collector power supply voltage E.sub.c =2 V, a load resistance R.sub.L =200.OMEGA., and an input signal voltage to turn transistor Q off V.sub.off =0.5 V. An input signal voltage to turn transistor Q on V.sub.on is given in Table 1. In type A, the first emitter layer has an impurity concentration N.sub.E of 3.times.10.sup.16 cm.sup.-3 and a thickness W of 1 .mu.m. J.sub.E and J.sub.C denote current densities of the emitter and the collector, respectively.
The opposite conclusion described above is based on the following reasoning. In general, in order to switch the bipolar transistor at a high speed, the emitter and collector current densities must be set from 10.sup.3 to 10.sup.4 A/cm.sup.2 or higher. This fact is apparent from various experiments and from analytical results using the numerical analysis model. With the transistor of type A having the first emitter layer of a low-impurity concentration, the capability to inject carriers from the emitter to the base is smaller than that of type B. In order to obtain predetermined emitter and collector current densities, a high forward-bias voltage must be applied across the emitter-base junction. Under this operating condition, excess carriers are stored in the thick first emitter layer and the collector layer. As a result, the turn-off time is increased, and hence the propagation delay time is increased.
In fine, the emitter junction capacitance C.sub.JE of type A is smaller than that of type B, but the switching speed of type A is lower than that of type B. This result implies that both the emitter junction capacitance C.sub.JE and a total emitter capacitance C.sub.E =C.sub.JE +C.sub.DE must be considered as factors in determining the switching speed of the transistor, where C.sub.DE is known as the emitter diffusion capacitance, which is determined by the excess carrier charge. Since the thick first emitter layer of a low-impurity concentration is formed in the conventional heterostructure bipolar transistor, C.sub.DE is greater than C.sub.JE. An effect of a small C.sub.JE cannot be observed under the influence of C.sub.DE with respect to the switching speed.
It is apparent that type B is preferred to type A from the point of view of switching speed. However, since type B has the emitter layer of a high-impurity concentration directly formed on the base layer, the breakdown voltage of the emitter junction is very low. The breakdown of a p-n junction mainly occurs due to the avalanche phenomenon. Even if the avalanche phenomenon is prevented, however, breakdown can also occur due to the tunneling effect. In particular, in the heterostructure bipolar transistors, a current caused by the tunneling effect consists of a component controlled by a number of interface levels generated at the heterojunction interface, in addition to a component determined by the direct transition of carriers between energy bands. For this reason, in practice, a tunneling current often becomes greater than a theoretical value. As a result, the emitter-junction breakdown voltage becomes very low.
In "A Depletion Stop Double Base Phototransistor: A Demonstration of a New Transistor Structure" by C. Y. Chen et al., IEDM 81, PP. 267-270, are disclosed a double base structure for phototransistors which is composed of a lightly doped layer near the emitter junction and a heavily doped layer near the collector junction, and a double-emitter double-base structure which is composed of lightly doped base and emitter layers near the emitter junction, heavily doped base and emitter layers far from the emitter junction. However, no specific formula to define the layer concentration and thickness is given. In addition, in the prior art device, a problem caused by excess carrier charge is left unsolved. It is noted that the phototransistor differs from the bipolar transistor of the present invention.